1. Field of the Invention
This invention relates to a method for producing embossed thermoplastic sheet materials and to the embossed thermoplastic material produced thereby.
2. Description of the Prior Art
Embossed plastic film or sheet material has come into widespread use in many fields. One particularly large scale use of embossed thermoplastic sheet material is that of disposable articles such as hospital pads and drapes, wearing apparel and disposable diapers. Embossed film is also finding increased use in the packaging field, for example, as bags and overwraps for articles such as clothing, etc., and for shopping bags. In order to fulfill the requirements established by the end use of embossed film, it is desirable that the film have little or no tendency to curl when run through fabricating machines, particularly those used for manufacture of disposable clothing articles, e.g., diapers. In some cases, it is important that the embossed thermoplastic film be soft and flexible and have the proper pattern and embossment depth in order to provide the desired "hand" or clothlike feel for the thermoplastic embossed material. Additionally, it is desired that the embossed thermoplastic material for many uses have as low a surface gloss as possible in order to simulate woven, clothlike fabrics. Further, embossed thermoplastic materials must meet the minimum physical requirements necessary for the films to be handled in high speed, automatic, fabricating machinery, i.e., it should have suitable modulus, tensile strength and impact strength.
Heretofore, embossed thermoplastic films such as polyethylene, polypropylene, polybutene-1, polyvinyl chloride, and other flexible thermoplastics normally extruded into film have been made by various methods. One method used to prepare embossed thermoplastic film is to extrude the thermoplastic material, e.g., polyethylene, from a conventional slot extrusion die onto a continuously moving, smooth, cooled casting surface, e.g., a chill roll. The engraved pattern may be applied to the chill roll and the film pressed to the roll while in the amorphous or molten stage by press rolls. Alternatively, the chill roll may be very smooth and the desired pattern in the film may be impressed into the film on the chill roll by means of an engraved or machined embossing roll which is pressed against the film and the chill roll to impress the pattern into the film as it is cooled on the chill roll. The softness of embossed film produced by chill casting is directly related to the density of the polyethylene resin used. In order to obtain different degrees of softness or stiffness, it is necessary to use a number of polyethylene resins having different densities. Thus, if it is desired to produce a relatively stiff embossed film, it is necessary to use more expensive polymers having high densities as the feed material to the slot die. Additionally, it is difficult to change many of the physical characteristics desired in the film when using the foregoing slot die-chill cast roll embossing technique. The embossed films made by the above described process often produce poor roll conformation, i.e., have hard or soft spots, wrinkle or sag due to the inability to distribute zones of thickness variation across the width of the roll of embossed film. Embossed film rolls of poor conformation produces problems when running the film through fabricating machines or through a film printing apparatus.
As an example of a method and apparatus for producing film according to the foregoing slot die-chill cast roll technique is shown in U.S. Pat. No. 3,374,303.
Another technique used for embossing plastic film heretofore has been the utilization of a heated engraved embossing roll used in conjunction with a backup roll. The preformed strip of thermoplastic film, normally at room temperature, is passed between the nip of a heated engraved roll and a backup roll and is embossed by being heated while in contact with the heated, engraved roller. The resultant embossed film usually has a very shallow and poorly defined pattern. An example of an apparatus and process for carrying out embossing of this type is shown in U.S. Pat. No. 3,176,058.
A third process for embossing thermoplastic film used heretofore has been to pass the film over a heated roll or a series of heated rollers in order to heat the film to a softened state and then to contact the film with an embossing roller and to press the film against the embossing roller by a backup roller. Normally, the embossing roller and the backup roller are cooled in order to freeze the embossed pattern into the film so that it may be immediately wound up into rolls, if desired. An apparatus and process for preparing an embossed film according to the foregoing is shown in U.S. Pat. No. 3,246,365.
One of the drawbacks when using the heated drum or roll method to apply heat to the film prior to embossing is the difficulty in heating the film sufficiently so that it will take a deep, permanent embossed pattern which is a true reproduction of the engraved surface of the engraving roll. If the film is heated sufficiently to obtain a very deep and permanent impression when pressed against the engraving roll, oftentimes the film will stick or adhere to the heating rolls, thus causing tears or slack or drape problems in handling the film between the heating rolls and the embossing rolls. Since the film cannot be heated by this method to the required temperature, this results in a loss of the density depression in the resin which is desired oftentimes to produce very soft, flexible film. Density depression produced in an embossed film is directly dependent on the quench rate applied to the film.
Thus, it can be seen that the film embossing art is in need of the process of the present invention whereby preformed thermoplastic film can be rapidly heated to a sufficiently high temperature, embossed, and then quickly cooled to obtain a density depression in film and thereby obtain a deep, permanent embossed pattern on the film and yet which will provide wide variation in control of the parameters which will permit variation in the physical properties of the embossed film, i.e., modulus, surface gloss, embossed pattern depth, tensile strength and impact strength.